MAC Sublayer - Winona State University
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Transcript MAC Sublayer - Winona State University
CS 313 Introduction to
Computer Networking &
Telecommunication
Medium Access Control Sublayer
Chi-Cheng Lin, Winona State University
Topics
Introduction
Channel Allocation Problem
Multiple Access Protocols
CDMA
2
Introduction
Broadcast networks
Key issue: who gets to use the channel
when there is competition
Referred to as
Multiaccess channels
Random access channels
MAC (Medium Access Control) sublayer
LANs
Wireless networks
Satellite networks
3
Channel Allocation Problem
Channel Allocation
Static
Dynamic
Performance factors
Medium access delay
Time between a frame is ready and the frame
can be transmitted
Throughput
#frames can be transmitted in unit time
interval
4
FDM
Static Channel Allocation
Bandwidth divided into N equal sized
portions for N users
Problems
#senders large
#senders continuously varies
bursty traffic
Discussion: #users > N ?
<N?
=N?
N times worse than all frames queued in
one big queue
5
Static Channel Allocation
TDM
Each user is statically allocated every Nth
time slot
Same problems as FDM
Under what circumstances are static
channel allocation schemes efficient?
6
Dynamic Channel Allocation
Key assumptions
1. Station model
Independent
Work is generated constantly
One program per station
Station is blocked once a frame has been
generated until the frame has been successfully
transmitted
2. Single channel assumption
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Dynamic Channel Allocation
Key assumptions
3. Collision Assumption
Collision:
Two frames are transmitted simultaneously,
overlapped in time and resulting signal garbled
Can be detected by all stations
No other errors
8
Dynamic Channel Allocation
Key assumptions
4. Time: either continuous or discrete (slotted)
Continuous
Frame transmission can begin at any instant
No "master clock" needed
Slotted
Time divided into discrete intervals (slots)
Frame transmissions begin at the start of a slot
#frames contained in a slot: 0 ?
1?
>1 ?
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Dynamic Channel Allocation
Carrier sense ("carrier" refers to
electrical signal): either Y or N
Yes
A station can check channel before transmission
If busy, station idle
Wired LANs
No
“Just do it"
Can tell if transmission successful later
Wireless networks, cable modems
10
Multiple Access Protocols
ALOHA
Carrier sense multiple access protocols
(CSMA)
CSMA w/ collision detection (CSMA/CD)
Collision-free protocols
Limited-contention protocols
11
ALOHA
Applicable to any contention system
System in which uncoordinated users are
competing for the use of a single shared
channel
Two versions
Pure ALOHA
Slotted ALOHA
12
Pure ALOHA
Let users transmit whenever they have data
to be sent
Colliding frames are destroyed
Sender can always find out destroyed or not
Feedback (property of broadcasting) or ACK
LANs: immediately
Satellites: propagation delay (e.g., 270msec)
By listening to the channel
If frame is destroyed
wait a random amount of time and retransmit
(why "random"?)
13
Pure ALOHA
Where are
the collisions?
14
Slotted ALOHA
Discrete time
Agreed slot boundaries
Synchronization needed
Performance
Which ALOHA has a shorter medium access
delay?
Which ALOHA has a higher throughput?
15
Performance of ALOHA
Slotted ALOHA can double the
throughput of pure ALOHA
Throughput versus offered traffic for ALOHA systems.
16
Carrier Sense Multiple Access (CSMA)
Protocols
Stations can listen to the channel (i.e.,
sense a carrier in the channel)
Types
1-persistent CSMA
Nonpersistent CSMA
p-persistent CSMA
17
Performance of MAC Protocols
Comparison of the channel utilization versus load for various
random access protocols.
18
CSMA w/ Collision Detection
(CSMA/CD)
Can listen to the channel and detect
collision
Stop transmitting as soon as collision
detected
Widely used on LANs (e.g., Ethernet)
Collision detection
Analog process
Special encoding is used
19
CSMA w/ Collision Detection
(CSMA/CD)
Conceptual model
3 states
Contention
Transmission
Idle
Minimum time to detect collision
determines time slot
Depends on propagation delay of medium
20
CSMA/CD Model
21
CSMA/CD Algorithm
Source: http://www.10gea.org/gigabit-ethernet/
22
Collision-Free Protocols
Model
N Stations: 0,1, ..., (N-1)
Question
Which station gets the channel after a
successful transmission?
Protocols
Bit-map (i.e., reservation) protocol
Token passing protocol
Example: Token ring
23
Collision-Free Protocol
Token
Station
Direction of
transmission
Token ring
Performance of Contention and
Collision-Free Protocols
Contention
Low load => low medium access delay :)
High load => low channel efficiency :(
Collision-Free
Low load => high medium access delay :(
High load => high channel efficiency :)
25
Summary of Channel Allocation
Methods/Systems
*
*
*
*
*
*
*
*
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* | Token Passing
| Contention-free protocol
|